1. Field of the Invention
The present invention relates generally to joining and sealing systems used in plastic pipelines of the type used for transporting fluids, potable water, sewage, electrical power lines, telephone and communication lines and, more specifically to such pipe systems which are assembled together with a special coupling which is resistant to separation in the presence of a wide range of external and internal forces including seismic forces.
2. Description of the Prior Art
Fluid joining and sealing systems for plastic, fluid conveying pipes are used in a variety of industries. The pipes used in such systems are typically formed from thermoplastic materials including polyolefins and PVC. In forming a joint between sections of PVC pipe, the spigot or male pipe end is inserted within the female or socket pipe end. An annular, elastomeric ring or gasket is typically seated within a groove formed in the socket end of the thermoplastic pipe. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint. Various types of sealing technologies have been employed to assure the sealing integrity of the pipe joint.
While such pipe joints may form adequate seals, problems have sometimes occurred in some instances, particularly where the pipe line is subjected to earth movement and other seismic type forces. The tendency of the connected pipe sections to pull apart while in service, when subjected to various sources of stress, can result in consequent damage to the pipeline and environment, as well as presenting a difficulty in locating the break for repair. Plastic pipe systems which do not include interlocking ends also present problems when there is a change in direction or the pipeline, or a change in diameter of the pipe sections, since unbalanced thrust forces may separate the pipe segments. Another situation arises when extreme tensile forces are exerted on the pipe string, such as in an earthquake. As a result, various specialized end connections and coupling devices have been employed utilizing, e.g., external clamps, sleeves or other auxiliary paraphernalia. Such devices are less than advantageous since they require extraneous parts and since the protruding clamp portions of the devices always present the danger of becoming accidentally struck when installed inside a casing or of causing the pipe to move more when the ground laterally flows as a result of an earthquake. External mechanical restraint mechanisms are usually subject to corrosion and deterioration over time. They are also often complicated in design and add to the labor cost in installing the mechanisms in the field. Because they usually require tightening bolts and the like, problems can occur in field installations.
One particular problem area is that of seismic joints or connections. In earthquake prone areas or areas with underground faults, a perpendicular crossing of the fault line subjects the pipeline mainly to bending forces. An oblique crossing subjects portions of the pipeline to compression forces and bending while other portions of the pipeline may be subjected to tension forces and bending. Where the pipeline lies parallel to the main length of the fault, portions of the pipeline are subjected to tension while other portions of the pipeline are subjected to compression. All of these forces may cause an unrestrained joint to separate, causing the connection to fail.
A need therefore exists for a plastic pipe coupling which would help to insure the integrity of a pipe joint, even in the presence of extreme seismic events, such as earthquakes and earth movement.
A need also exists for such a coupling which would be relatively simple in design and economical to manufacture.
A need also exists for such a seismic coupling for plastic pipe joints which can be easily adapted for PVC pipe systems and which provides a reliable seal as well as a restrained connection.